Charge sharing and the lost energy
"Charge sharing and the lost energy" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Energy stored in a capacitor" in Electrostatic Potential and Capacitance. A charged capacitor joined to an uncharged one: charge shares to a common potential and an energy bar shows the part lost as heat. NEET has asked about this idea 3 times in 10 years of papers. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Energy stored in a capacitor in Electrostatic Potential and Capacitance. Change the values and watch what happens.
What it shows
NCERT §2.15
A charged capacitor joined to an uncharged one: charge shares to a common potential and an energy bar shows the part lost as heat.
Controls
4 to try
- SliderC1 100–2000 pF
- SliderC2 0–2000 pF
- Sliderinitial voltage 10–200 V
- Buttonclose switch
Try this
Pick C2 so exactly half the stored energy is lost (C2 = C1), e.g. 900 pF at 100 V leaves 2.25 × 10⁻⁶ J.
Common mistake
Thinking energy is conserved because charge is.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: Capacitance, Electrostatic Potential and Capacitance (chapter)
In JEE Main
42 shifts analysed
48 questions from Electrostatic Potential and Capacitance across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §2.15
- Charging moves charge from the negative plate to the positive plate, which is at the higher potential, so external work is done at every step.
- When the charge is Q′ the voltage is Q′/C; moving a further δQ′ takes (Q′/C)δQ′. Adding these from 0 to Q gives W = Q²/(2C).
- Stored energy U = Q²/(2C) = ½CV² = ½QV. It is ½QV, not QV, because the voltage builds up from zero while the charge is moved.
- The energy can be pictured as stored in the field between the plates. For a parallel plate capacitor U = ½ε₀E² × Ad, where Ad is the volume of the gap.
- Energy density of an electric field u = ½ε₀E² (joules per cubic metre). This holds for any field, not only a capacitor's.
- Worked value: 900 pF charged to 100 V holds Q = 9 × 10⁻⁸ C and U = 4.5 × 10⁻⁶ J.
- Join that charged capacitor to an identical uncharged one: the charge shares equally, V halves to 50 V, and the total energy drops to 2.25 × 10⁻⁶ J. Charge is conserved; the missing half is lost as heat and electromagnetic radiation while charge flows.
Questions
5 answers
- What does the "Charge sharing and the lost energy" simulation show?
- A charged capacitor joined to an uncharged one: charge shares to a common potential and an energy bar shows the part lost as heat.
- What mistake does the "Charge sharing and the lost energy" simulation fix?
- A common mistake it clears up: thinking energy is conserved because charge is.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 times in 10 years of papers.
- Does JEE Main ask questions from Electrostatic Potential and Capacitance?
- Yes. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Energy stored in a capacitor" in Electrostatic Potential and Capacitance (Class 12 Physics).
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